Slow Axons Research Articles

The effects of temperature on a neuromuscular system of the crayfish,Astacus leptodactylus

1. Slow and fast contractions, as well as the corresponding excitatory junction potentials (ejp's) were recorded from the closer muscle (adductor of the dactylopodite) of the third walking legs, subjected to temperatures ranging from +6 °C to +32 °C. In addition, the effect of temperature on resting potential and resting tension was studied, and in several preparations we investigated the effect of temperature on the efficacy of the inhibitory axon. The experimental animals had been acclimated at 12 °C. 2. The membrane potential was found to increase with temperature. In the lower temperature range (e.g., between 6°C and 14–18 °C) the slope was usually steeper; the maximal rate of change was 2 mV/°C. 3. The amplitude of facilitated slow ejp's (10/s) changes only insignificantly over the temperature span from 6 °C to about 22 °C and declines at higher temperatures. Fast ejp's have maximal amplitudes at 20 °C and decline at lower and higher temperatures, the decline being insignificant, however, below 14 °C. 4. Facilitation rate also changes little with temperature; a significant increase occurs near 30 °C in the case of fast ejp's, and between 15 °C and 20 °C in the case of slow ejp's. Above 28 °C, facilitation of slow ejp's declines. 5. As the temperature is lowered, many muscles develop a contracture which may reach 10% of maximal tension at 6 °C. These temperature-induced contractures can be abolished by stimulation of the inhibitory axon. 6. The effectiveness of the inhibitory axon in reducing the contraction caused by the slow motor axon increases with decreasing temperature. 7. The time course of the decay of both slow and fast ejp's increases with falling temperature, particularly in the temperature range below 12 °C. 8. At a given frequency (1–30/s), both slow and fast contractions increased with falling temperature over the entire temperature range tested. This increased efficiency of the neuro-muscular system is attributed to the depolarization and the prolonged time course of the ejp's.

Properties of relay cells in cat's lateral geniculate nucleus: a comparison of W-cells with X- and Y-cells.

1. Observations are presented on the physiological properties of W-, X-, and Y-type relay cells in the cat's lateral geniculate nucleus (LGN). Emphasis is placed on the most recently recognized type, W-cells; data are presented on X- and Y-cells by way of comparison. 2. Seventy-seven W-cells were recognized on 70 microelectrode penetrations through the LGN. They resembled W-type retinal ganglion cells in their responses to visual stimuli. Tonic (on-center and off-center) W-cells, phasic (on-, off- and on-off center) W-cells, suppressed-by-contrast, and color-coded cells were recognized. 3. W-type relay cells also resembled retinal W-cells in their maintained activity and receptive field-center diameters. 4. W-type relay cells comprised 11.5% X-cells 48.4%, and Y-cells 22.3% of all LGN cells encountered on a reference sample of 62 electrode tracks. W-cells were found in laminae C, C1, and C2, comprising 36.5% of the sample in these laminae, but were not encountered in laminae A or A1. X- and Y-cells were found in laminae A, A1, and C. Within lamina C there was a tendency for X- and Y-cells to be located dorsal to W-cells. There was thus a substantial dorsoventral segregation of W-cells from X- and Y-cells. W-cells being found in the ventral parvocellular component of the dorsal LGN. 5. Cells considered to be W-type relay cells were shown to respond to electrical stimulation of the optic nerve and chiasm at latencies which were longer than those of X- and Y-cells, and were consistent with their receiving monosynaptic input from retinal W-cells. Geniculate W-cells of all subtypes were activated antidromically from the visual cortex. Their antidromic latencies were, on the average, longer than for Y- or X-cells, indicating that W-type relay cells had slower axons as well as slower retinal afferents, than X- or Y-cells. 6. The visual cortex thus appears to receive input from all three major types of retinal ganlion cells (W-, X-, and Y-cells) relayed separately, in parallel, by different groups of relay cells.

Re‐innervation of twitch and slow muscle fibres of the frog after crushing the motor nerves.

1. The conduction velocities of individual motor axons innervating twitch and slow muscle fibres of the frog were determined by intracellular recording of junctional potentials elicited by stimulating the motor nerves at two different points. 2. In normal pyriformis muscles twitch and slow fibres were found to be innervated by two distinct populations of motor axons. Twitch fibre axons conducted at 10-18-7 m/sec, while the conduction velocities of slow fibre axons ranged from 0-5 to 5 m/sec (at 7-9 degrees C). The thresholds for electrical stimulation were significantly lower in the fast than in the slow axons population. 3. Following denervation by crushing the sciatic nerve fast axons which re-innervated the muscle had lower conduction velocities than normal but could still be identified. These lower conduction velocities were measured proximal to the site of the crush and did not recover over a period of 446 days. 4. Fast motor axons regenerated more quickly than slow axons and re-innervated twitch as well as slow muscle fibres non-selectively. About 1 month later slow axons re-established synaptic contacts with slow (and some twitch) muscle fibres. Simultaneous re-innervation by fast and slow motor axons was occasionally observed in slow muscle fibres. Finally, the slow muscle fibres were innervated by slow axons only, while synapses of fast axons could no longer be found in this type of muscle fibre. 5. Action potentials were observed in denervated as well as in re-innervated slow muscle fibres; they disappeared as re-innervation progressed. 6. It is concluded that non-selective re-innervation of slow muscle fibres is present in the frog; it is, however, a transient phenomenon followed by restoration of the original innervation pattern.

Topographic studies on medial reticular nucleus.

Several distinct classes of neurons have been identified in the medial reticular nucleus of the medulla and pons and in proximity thereto. Neurons projecting down the spinal cord comprised the principal class with two subclasses according as the neurons did or did not receive monosynaptic inputs from the fastigial nuclei of the cerebellum. Two other classes were recognized accordings as they projected to the cerebellum or rostrally to the mesencephalon. Topographic planar maps giving the location of these neurons have been constructed by exploring the nucleus with series of microelectrode tracks in parasagittal or in transverse planes. The different classes of neurons were not arranged in large discrete nuclei. In part they appeared to be randomly distributed, but many colonies of one or another class of neurons could be recognized with 3-11 neurons in zones with dimensions of a millimeter or so. Because of the limitations of sampling by microelectrode tracks at spacings of 0.5 mm, single colonies might have an actual population of 100 or more. Many of the class of neurons projecting to the cerebellum were in the region of the perihypoglossal nucleus. However, almost as many were located deep in the medial reticular nucleus. None was found at the pontine level. Reticulospinal neurons with fast axonal conduction velocities tended to be located dorsally to those with slow velocities. Correlation with the findings of Ito et al. leads to the conjecture that the neurons with fast axons are excitatory, while those with slow axons are primary inhibitory neurons. There is a brief reference to the problems raised by the admixture of the various neuronal classes, there being discrete colonies immersed in a scattered arrangement of all classes.

The mode of synaptic linkage in the cerebro-ponto-cerebellar pathway of the cat. II. Responses of single cells in the pontine nuclei

Extracellular and intracellular recordings were made from single cells in the pontine nuclei (PN) of the cat. PN cells were identified by antidromic invasion from the cerebellum by stimulating either the brachium pontis (BP) or the white matter near the cerebellar nuclei. The cerebrally-induced impulses excited PN cells postsynaptically with a monosynaptic latency. Both fast and slow conducting cortical fibres contributed to the corticopontine excitation, so that the latency varied over a wide range. Measurements of the latencies for antidromic and corticopontine excitation and of the distances between stimulated sites permitted the calcuation of conduction velocities of PN cell axons and of their cortical input fibres. PN cells with fast conducting axons received convergence from both fast and slow cortical fibres, whereas PN cells with slow axons were innervated only by slow cortical fibres. The majority of PN cells were also excited by stimulating the medullary pyramid through collaterals of the pyramidal tract. Evidence of abundant pyramidal collaterals was provided by the collision technique. The functional role of the PN is discussed in connection with the cerebro-cerebellar loop circuits.

Neuromuscular analysis of closing in the dimorphic claws of the lobster Homarus americanus.

AbstractThe mechanical and electrical activity of the closer muscle is compared in the cutter and crusher claws of the lobster Homarus americanus. Two excitor axons innervate the closer muscle. In the cutter the fast axon produces twitch contractions in response to single stimuli while its counterpart in the crusher gives twitches only to paired stimuli. The twitches fatigue more rapidly in the cutter than in the crusher. The slow axon in both claws produces tonic contractions which are fatigue resistant. The electrical events in the muscle fibers correlate closely with the mechanical activity. Thus the cutter fast EPSPs show poor facilitation and fatigue rapidly while the slow EPSPs have high facilitation values and are less prone to fatigue. The slow and fast axon EPSPs in the crusher are qualitatively similar to their counterparts in the cutter. In both claws the postsynaptic response was considerably enhanced by paired stimuli.

Neuromuscular physiology of the longitudinal muscle of the earthworm, Lumbricus terrestris. II. Patterns of innervation.

ABSTRACT Patterns of innervation of the longitudinal muscle of the earthworm, Lumbricus terrestris, were examined electrophysiologically. The longitudinal musculature of a segment is innervated by relatively few axons, a fast and slow axon being present in segmental nerve I and in the double nerve, segmental nerve II–III. Single-pulse stimulation of the fast axon produces large external muscle potentials and small twitch-like contractions, which with repetitive stimulation are antifacilitating. Repetitive stimulation of the slow axon produces large, slowly developing and sustained mechanical responses, with electrical and mechanical responses showing summation and facilitation. The amplitude and time course of slow mechanical responses are related to the frequency of stimulation. Individual longitudinal muscle fibres are innervated by either the fast or slow axon in a segmental nerve, or by both fast and slow axons. No evidence was found for peripheral inhibitory innervation of the longitudinal muscle.

Identification of the somata of common inhibitory motoneurons in the metathoracic ganglion of the cockroach.

The somata of the three common inhibitory neurons known to innervate many muscles of the metathoracic legs of the cockroach have been identified by allowing cobalt chloride to diffuse up the cut axons of these cells and by precipitating the cobalt with ammonium sulfide. The black precipitate of cobalt sulfide allowed the somata and axonal pathways of these three cells to be clearly visualized. All three somata are located near the midline on the ventral surface of the ganglion. The most posterior of these somata is just contralateral and sends branches directly out lateral nerves 3, 4, and 6, and indirectly into nerve 5 via a small connective between nerves 4 and 5. The other two somata each send a branch out only nerve 5. The somata of the fast and slow excitatory motor axons to the coxal depressor muscles were also identified in this study.

Insect Neuromuscular Synapses

Miniature end-plate potentials were used in studying several aspects of the neuromuscular systems in the cockroach femur. The similar sizes and time courses of miniatures associated with fast and slow type excitatory axons suggest that they employ the same transmitter. There is other evidence also indicating that the essential difference between these two excitatory systems is in the number of packets of transmitter released per nerve impulse rather than different transmitter substances. From the shapes of miniatures it was suspected that typical muscle fibers might have a branching structure. This was confirmed by histological examination, intracellular stimulation, and intracellular dye injection. That inhibitory transmission is quantal is indicated by hyperpolarizing miniatures which occur at random time intervals. Inhibitory transmission can be made to fail and recover in a stepwise manner by manipulating the Ca/Mg ratio. In studies of toxins which affect transmitter release at vertebrate motor end-plates, botulinal toxin was found to be without effect at either excitatory or inhibitory junctions in cockroach muscle. However, black widow spider venom acted as it does in vertebrates, promoting massive release of transmitters and then permanent inactivation of the junctions.

Steps in production of motoneuron spikes during rhythmic firing.

Steps in production of motoneuron spikes during rhythmic firing.

Neuromuscular physiology of the strike mechanism of the mantis shrimp, Hemisquilla

AbstractThis study was planned to determine the neuromuscular basis of the very rapid strike movement, lasting only 5–10 ms, used by mantis shrimps to capture prey or defend themselves. The raptorial second thoracic append‐ages which are responsible for the strike action are unfurled by four muscles in the merus, two extensors and two flexors. All were found to be specialized for slow not fast contraction. The largest extensor muscle has a twitch time of 400 ms, reaches peak tetanic tension in 700 ms and takes one second to relax completely from this. It receives two “slow” and one “fast” motor axons distributed uniformly throughout the muscle. An overshooting spike occurs in each muscle fiber from the first activation of a train in the fast axon, but many seconds must elapse before the first muscle fiber will again respond with a spike. The smaller, medial extensor receives one fast axon which may give rise to overshooting spikes and an inhibitor whose effects appear to be minor; its muscle fibers are only weakly electrically excitable. The contraction and relaxation rates are slow, as are those of the medial flexor which receives two slow axons. The lateral flexor is divisible into two parts; a proximal region comprised of electrically passive fibers innervated solely by a slow axon and a distal region of spiking fibers innervated by a fast axon. The rapid strike action is explained not in terms of the rapid contraction of these muscles but in their operation of a mechanical device in the form of a “click‐joint,” described previously.

Innervation of coxal depressor muscles in the cockroach, Periplaneta americana.

ABSTRACT The innervation of the mesothoracic and metathoracic coxal depressor muscles has been investigated using intracellular microelectrodes. Five motor axons have been found to innervate the coxal depressor muscles; the largest is a fast axon, the second largest a slow axon and the three smallest are all inhibitory axons. The fast axon innervates the anterior and posterior coxal depressor muscles as well as parts of the coxal branches of the main leg depressor muscle. The slow innervates all fibres in the coxal branches of the main leg depressor muscle. The three inhibitory axons innervate only those parts of the coxal branches of the main leg depressor muscle which are not innervated by the fast axon. The three inhibitory axons were found to be branches of three different common inhibitory motoneurones.

Discharge patterns of coxal levator and depressor motoneurones of the cockroach, Periplaneta americana.

ABSTRACT Observation of movements of the metathoracic legs of the cockroach before and after section of peripheral nerves allowed identification of muscles involved in flexion and extension of the femur. Extracellular recordings from the nerves to these coxal muscles show that during rhythmic leg movements bursts of activity in a number of levator motor axons were strongly reciprocal and generally non-overlapping with those of a slow depressor motor axon. These reciprocal patterns persisted after removal of all sensory input from the legs. The durations of levator bursts were relatively constant compared to those of the depressor, corresponding to the behavioural observations on leg protraction time. The pattern was asymmetric: levator bursts could be generated without depressor activity, but never the reverse. No evidence was found for inhibitory collateral pathways between antagonisti motoneurones. It is proposed that levator motoneurones are driven by a group of bursting interneurones which simultaneously inhibit the ongoing depressor activity.

Insect visceral muscle: Neural relations of the proctodeal muscles of the cockroach

The muscles of the proctodeum and their innervation are described for the cockroach, Periplaneta americana. The proctodeal nerve arises bilaterally as a small branch of nerve XI. Two populations of efferent nerve fibres were distinguished by ‘fast’ and ‘slow’ nerve spikes. Fast spikes (0·5 m/sec) evoked muscle PSP's and, thus, were conducted by motor axons. Motor axons were stained characteristically with methylene blue; they originated in the sixth abdominal ganglion and terminated in fine, varicose endings; associated ganglion cells were not observed. Slow, efferent nerve spikes (0·2 m/sec) did not evoke a muscle response; the nature of the slow axon is uncertain. Afferent activity was recorded from the proctodeal nerve and sensory nerve cells were present below the circular muscle layer. The proctodeum was enveloped by a ‘periproctodeal net’, a syncytium of polynucleated fibres that is innervated by motor axons; it does not appear to constitute a peripheral nerve net.

Neuromuscular synapses in the cockroach extensor tibiae muscle

The fine structure of fast-axon and slow-axon neuromuscular synapses was investigated in the extensor tibiae muscle of the cockroach ( Periplaneta americana), using muscle fibres which were found to be innervated by only one of the two motor axons. Synapse-bearing terminals of the fast axon are larger and form more synaptic contacts than those of the slow axon. There is no difference in the synaptic vesicles of the two axons. Synaptic vesicles are often seen in association with axon filaments or tubules. Extra-axonal vesicles are sometimes found in close association with the axons.

Polyneuronal innervation of the fast muscles of the marine teleost Cottus scorpius L.

ABSTRACT Histological and electrophysiological studies of the spinal nerves, nerve roots and muscles of the abdominal wall of the marine teleost Cottus scorpius have been undertaken to determine the extent and nature of polyneuronal innervation of the fast muscles. Spinal nerves at proximal and distal levels, and the dorsal roots, contain axons in a single mixed population with a mean diameter of 2–4 μm., while the ventral roots contain axons in two diameter classes with means at 4–6 and 12–14 μm. Between 8 and 22 distributed nerve terminations were counted on fifty-two teased intact single muscle fibres stained for acetylcholinesterase activity. The average distance between the terminals is 0·64 mm. (range 0·094–2·050 mm.). The compound action potential of the nerve comprises two principal peaks with conduction velocities of 17·0–23·8 m./sec. and 1·5–12·2 m./sec. at 10–12°C. Fast muscle fibres gave two types of electrical response-all-or-none spike potentials that are propagated with a conduction velocity of c. 1·1 m./sec. at 10–12° C., and quantized distributed junction potentials. The electrical properties of the nerves and roots suggest that the fast muscles are innervated by a single class of fast axons and possibly by a few slow axons. Simultaneous recordings of nerve and muscle activities were made at different stimulus intensities. In all cases muscle responses were correlated with the first peak of the compound action potential, and appeared with the same or only slightly different latencies. Each muscle fibre is shown electrophysiologically to be polyneuronally innervated by 2–5 axons from a single spinal nerve, and to receive a similar axonic complement from each of four spinal nerves. Polyneuronal innervation of the muscle fibres by 8–22 different axons in the absence of multiterminal innervation is postulated.

The Action of the Eyecup Muscles of the Crab, Carcinus, During Optokinetic Movements

ABSTRACT The actions of the nine eyecup muscles of the crab during horizontal optokinetic movements are described. Each muscle includes a wide spectrum of fibre types, ranging from phasic, with sarcomere lengths of 3–4 μm., through intermediate, to tonic fibres with sarcomeres of 10–12 μm. Each muscle receives at least one slow and one fast motoneuron, but no inhibitory supply. The slow axons predominantly innervate the tonic muscle fibres while the fast axons innervate the phasic ones. Slow movement and the position of the eyecup in space are controlled by the frequency of slow motoneuron discharges. All muscles collaborate at every position. The phasic system is recruited during rapid eyecup movements of large amplitude. In optokinetic nystagmus the exact form of the impulse sequences are described for each muscle. They are the consequence of a visually driven central programme which takes no account of the movement which it generates. Movements in opposite directions involve different central programmes; the one is not merely the reverse of the other. There is no effective proprioceptive feedback from the eyecup joint or from muscle tension receptors.

Motor innervation, motor unit organization and afferent innervation of m. extensor digitorum communis of the baboon's forearm

1. One hundred and fifty efferent axons innervating m. extensor digitorum communis (EDC) were isolated in filaments of C7 and C8 ventral roots of baboons. Conduction velocities were measured antidromically by stimulating the muscle nerve and recording from the filaments, and fell into two groups: a fast (49-84 m/sec) and a slow (22-41m/sec), presumably fusimotor group. The threshold for these latter axons exceeded the strength needed to elicit the maximal motor twitch.2. Stimulation of ventral root filaments containing slow axons produced no contractile tension in EDC.3. Stimulation of ventral root filaments containing fast-group axons elicited all-or-nothing twitches of motor units of EDC. The twitch tensions of 66.3% of the units were < 2.0 g wt.; only 8.7% were > 5.0 g wt. Tetanus-twitch ratios were 1.4-4.7 in a sample of 14 units. Contraction times were between 15 and 35 msec in 97% of the units. There was no correlation between contractile properties and axonal conduction velocity.4. Afferent volleys from the stimulated EDC nerve were recorded from C6 or C7 dorsal roots. The threshold was below the threshold for a just-detectable motor twitch in ten out of eleven baboons. Conduction velocity of the earliest component of the muscle afferent volley was 67-83 m/sec.5. The conduction velocities of twenty-eight spindle afferents, identified by their responses to linear stretches of EDC and by their unloading by maximal twitches, were all < 70 m/sec. Higher dynamic sensitivity tended to be associated with higher conduction velocity.

Inhibition in crustacean phasic neuromuscular systems

1.1. The neuromuscular systems of the closer muscle of the crab, Pachygrapsus crassipes Randall, and of the deep abdominal extensor muscler of the crayfish, Procambarus clarki (Girard), and the rock lobster, Panulirus interruptus (Randall), were investigated with respect to the effectiveness of the inhibitory axons on excitatory processes and on the muscle fibre membranes.2.2. In the closer muscle of Pachygrapsus, the muscle fibres showing large electrical responses to the slow axon also showed large inhibitory potentials. Both the slow and, when present, the fast electrical responses in these fibres were greatly attenuated by inhibition.3.3. The muscle fibres giving large electrical responses to the fast axon showed no signs of inhibitory innervation.4.4. In the abdomial extensor muscle preparations, inhibitory innervation was present and inhibitory stimulation generally caused a sizeable increase in membrane conductance of the muscle fibres, as did γ-aminobutyric acid. The active membrane responses in these fibres were often selectively eliminated or reduced by inhibition.5.5. The excitatory junctional potentials were not markedly attenuated by inhibition, although their rate of decay was increased. No evidence for a presynaptic inhibitory mechanism was found in these muscles. All of the inhibitory effects could be attributed to a post-synaptic increase in membrane conductance.6.6. Inhibitory and γ-aminobutyric acid actions were enhanced by lowering the temperature.

Some aspects of the efferent control of walking in three cockroach species

We have studied the efferent commands to muscles during walking in three widely different species of cockroach and find no significant differences between them in the patterns of motor output. Several different patterns can be employed by the same species to get the same stepping rate, particularly at low speeds. As stepping rate increases, the following general trends can be seen. (a) There is recruitment of additional units, first of slow axons and then of fast. (b) The reciprocal relationship between antagonist muscles becomes less obvious, particularly with respect to the slow axon system. (c) At extremely high stepping rates the number of fast potentials becomes greatly reduced.